1. Field of the Invention
The present invention relates to an optical glass composition, a preform and an optical element. In particular, the present invention relates to: an optical glass composition suitable as the material of an optical element such as a lens element contained in a shooting lens system of a digital still camera or a digital video camera (simply referred to as a digital camera, hereinafter); a preform used in producing this optical element by press molding; and this optical element.
2. Description of the Background Art
In recent years, in digital cameras, various types ranging from common types to high-class types have been proposed in accordance with the consumers' needs. In such digital cameras, regardless of whether they are of common type or of high-class type, thickness reduction is strongly desired for improving portability. In order to achieve thickness reduction in a digital camera, reduction is indispensable in the thickness of the shooting lens system which occupies a relatively large volume.
For the purpose of thickness reduction in a shooting lens system, reduction in the number of lens elements is effective. Nevertheless, in recent years, increasingly high optical performance is required in shooting lens systems. Thus, reduction in the number of lens elements is reaching a limit, and hence remarkable further reduction is unexpectable. Accordingly, for the purpose of thickness reduction in a shooting lens system, thickness reduction becomes necessary in the individual lens elements contained in the shooting lens system.
For the purpose of thickness reduction in a lens element, it is effective to increase the refractive index of the glass material that forms the lens element. As examples of such a glass material having a high refractive index, optical glass is described in Japanese Laid-Open Patent Publication No. 2005-239476 and Japanese Laid-Open Patent Publication No. 2005-179142.
In an imaging lens system, aberration compensation is necessary. Then, in general, the aberration is compensated by variously combining the optical indices such as the refractive indices (nd) and the dispersions (νd: Abbe numbers) of the lens elements and the shapes of the lens elements. Thus, aiming at thickness reduction, when glass having a high refractive index is used merely abundantly, the aberration cannot be compensated successfully.
Among such imaging lens systems, in a zoom lens system, intervals between a plurality of lens units are changed so that variable magnification is achieved. Thus, aberration compensation like chromatic aberration compensation need be performed to an extent in each lens unit. Accordingly, each lens unit need contain at least two lens elements consisting of a lens element having positive optical power and a lens element having negative optical power. Nevertheless, from the viewpoint of aberration compensation, certain constraints are present on the optical indices such as the refractive indices (nd) and the dispersions (νd: Abbe numbers) of such two lens elements.
Japanese Laid-Open Patent Publication No. 2005-239476 discloses optical glass having a refractive index as high as exceeding 2.00. When such optical glass is used for the lens element having positive optical power in the lens unit of the zoom lens system, a high effect is obtained in reducing the thickness of the lens unit. Nevertheless, a strong tendency is present that in a refractive index range as high as exceeding 2.00, the Abbe number (νd) becomes low. Thus, for the lens element having negative optical power, optical glass is desired that has a relatively high refractive index and a high Abbe number. Specifically, for the lens element having negative optical power, optical glass is necessary that has a refractive index (nd) to the d-line falling within a range of 1.88 or higher and 1.92 or lower and an Abbe number (νd) to the d-line falling within a range of 33 or higher and 37 or lower. Further, in some cases, in addition to the constraints on these optical indices, excellent heat characteristics are required in the optical glass at the time of heat treatment in producing the optical element. Nevertheless, these Japanese Laid-Open Patent Publication No. 2005-239476 and Japanese Laid-Open Patent Publication No. 2005-179142 do not describe optical glass that has a refractive index (nd) to the d-line falling within a range of 1.88 or higher and 1.92 or lower and an Abbe number (νd) to the d-line falling within a range of 33 or higher and 37 or lower. Further, these documents do not describe optical glass that has a refractive index to the d-line and an Abbe number to the d-line falling within the above-mentioned ranges and also has excellent heat characteristics.